DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 409 
anterior axial mesoblast is thickened. It is continuous behind 
with the hypoblastic ridge, which it joins at a slight angle, and 
in front with the extra-embryonic hypoblast. 
In mesial longitudinal sectious the hypoblastic ridge extends 
backwards to the anterior end of the primitive streak, imme- 
diately in front of which it is considerably thickened, and 
becomes continuous with the epiblast (fig. 18). There is at 
this stage no trace of the neurenteric canal, but the more 
regular arrangement of the nuclei in both hypoblast and epi- 
blast in front of the primitive streak, and their less regular 
arrangement in the streak itself, clearly indicate the former 
position of the canal. 
The peculiar relations of the mesoblast of the anterior por- 
tion of the embryonic area, in front of the hypoblastic ridge, 
are most clearly revealed by transverse sections. Beneath the 
epiblast the hypoblast is bent towards the yolk-sac (fig. 18 C). 
The mesoblast extends across the mouth of the bay-shaped de- 
pression, and is attached laterally to the hypoblast. Beyond 
the marginal attachments of the anterior portion of the em- 
bryonic mesoblast small mesoblastic islets are found in the 
extra-embryonic area(M/J.). These are the first rudiments of 
the blood islets, and they are not connected with the embryonic 
mesoblast. 
Further back, at the anterior end of the hypoblastic ridge 
(fig. 18 A), the anterior axial mesoblast terminates in the 
middle line, but it becomes continuous by its lateral margins 
with the paraxial mesoblast of the anterior portion of the em- 
bryonic region (fig. 18D). The paraxial mesoblastic plates 
are widely separated anteriorly, for a distance of 40 mw, by the 
broad anterior extremity of the hypoblastic ridge, and late- 
rally they are continuous with the hypoblast, from which they 
seem to be receiving many new nuclei. In the interior of the 
plates, but nearer their outer than their inner extremities, 
cavities are developed (fig. 18 D). 
More posteriorly the inner ends of the paraxial mesoblastic 
plates approach nearer to each other, simultaneously with the 
narrowing of the hypoblastic ridge, and they assume a trian- 
